The demand for weight reduction in the automotive industry has led to the development and implementation of lightweight materials, and related manufacturing processes and tools. The growing concern for occupant safety also leads to the adoption of materials which improve the integrity of the vehicle during a crash while also improving the energy absorption.
A process known as Hot Forming Die Quenching (HFDQ) uses boron steel sheets to create stamped components with Ultra High Strength Steel (UHSS) properties, with tensile strengths up to 1,500 MPa. The increase in strength allows for a thinner gauge material to be used, which results in weight savings over conventionally cold stamped mild steel components.
Typical vehicle components that may be manufactured using the HFDQ process include: door beams, bumper beams, cross/side members, A/B pillar reinforcements, and waist rail reinforcements.
Hot forming of boron steels is becoming increasingly popular in the automotive industry due to their excellent strength and formability. Many structural components that were traditionally cold formed from mild steel are thus being replaced with hot formed equivalents that offer a significant increase in strength. This allows for reductions in material thickness (and thus weight) while maintaining the same strength. However, hot formed components offer very low levels of ductility and energy absorption in the as-formed condition.
In order to improve the ductility and energy absorption in key areas of a component such as a beam, it is known to introduce softer regions within the same component. This improves ductility locally while maintaining the required high strength overall. By locally tailoring the microstructure and mechanical properties of certain structural components such that they comprise regions with very high strength (very hard) and regions with increased ductility (softer), it may be possible to improve their overall energy absorption and maintain their structural integrity during a crash situation and also reduce their overall weight. Such soft zones may also advantageously change the kinematic behavior in case of collapse of a component under an impact.
Known methods of creating regions with increased ductility (soft zones) in vehicle structural components involve the provision of tools comprising a pair of complementary upper and lower die units, each of the units having separate die elements (steel blocks). The die elements are designed to work at different temperatures, in order to have different cooling rates in different zones of the part being formed during the quenching process, and thereby resulting in different material properties in the final product (soft areas). E.g. one die element may be cooled in order to quench the corresponding area of the component being manufactured at high cooling rates and by reducing the temperature of the component rapidly. Another neighboring die element may include heating elements in order to ensure that the corresponding portion of the component being manufactured cools down at a lower cooling rate, and thus remaining at higher temperatures than the rest of the component when it leaves the die.
One problem related to this sort of manufacturing is that where the die elements working at different temperature touch each other, a large temperature differential may be present, which creates a heat flow from a warm die element to a cold die element. The warm die element thus becomes slightly colder and the cooler die element becomes slightly warmer. The result may be that a relatively wide transition zone is created between a soft zone and a hard zone of the component. The behavior and characteristics of the component may thus be less well defined.
One solution to this problem may be to physically separate and thermally insulate die elements from each other e.g. by providing an idle gap in between them and/or by providing an insulating material in the gap. Document U.S. Pat. No. 3,703,093 describe such methods and tools. Manufacturing defects e.g. wrinkles or other irregularities in the final formed component may thus appear in those areas of the product that are not properly supported by or contacted by die elements.
Other known methods create regions with increased ductility by heating with a laser. But these methods are rather slow and cumbersome as laser heating is carried out after a HFDQ process.
It is an object of the present disclosure to provide improved tools for manufacturing hot-formed vehicle structural components with regions of high strength and other regions of increased ductility (soft zones).